Low-temperature partial magnetic self-reversal in marine sediments by magnetostatic interaction of titanomagnetite and titanohematite intergrowths

Abstract

S U M M A R YWith various low-temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the Argentine continental slope near the Rio de la Plata estuary, a so far unreported style of partial magnetic self-reversal has been detected. In these sediments the sulphate-methane transition (SMT) zone is situated at depths between 4 and 8 m, where reductive diagenesis severely alters the magnetic mineral assemblage. Throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions. In the SMT zone (titano−)magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium-rich titanohematite, originating from high temperature deuteric oxidation within the volcanic host rocks. These abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy. Warming of field-cooled and zero-field-cooled low-temperature saturation remanence displays magnetic phase transitions of titanium-rich titanohematite below 50 K and the Verwey transition of magnetite. A prominent irreversible decline characterizes zero-field cooling of room temperature saturation remanence. It typically sets out at ∼210 K and is most clearly developed in the lower part of the SMT zone, where low-temperature hysteresis measurements identified ∼210 K as the blocking temperature range of a titanohematite phase with a Curie temperature of around 240 K. The mechanism responsible for the marked loss of remanence is, therefore, sought in partial magnetic self-reversal by magnetostatic interaction of (titano-)magnetite and titanohematite. When titanohematite becomes ferrimagnetic upon cooling, its spontaneous magnetic moments order antiparallel to the (titano-)magnetite remanence causing an drastic initial decrease of global magnetization. The loss of remanence during subsequent further cooling appears to result from two combined effects (1) magnetic interaction between the two phases by which the (titano-)magnetite domain structure is substantially modified and (2) low-temperature demagnetization of (titano-)magnetite due to decreasing magnetocrystalline anisotropy. The depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments. Typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics. Low-temperature remanence cycling is, therefore, proposed as a diagnostic method to magnetically characterize such alteration (palaeo-)environments.

Magnetic particle characterization in the Seine river system: Implications for the determination of natural versus anthropogenic input

“…The RT‐SIRM curve of Andrésy shows furthermore a clear continuous remanence decrease in two steps, first between 260 and 200 K and second between 200 K and the Verwey transition. Similar curves were published for natural mixtures of titanomagnetite and titanohematite by, e.g., Garming et al [2007].…”

Magnetic petrology of equatorial Atlantic sediments: Electron microscopy results and their implications for environmental magnetic interpretation

“…Specular hematite encrustations were observed at all sites and may even stabilize the more vulnerable magnetite core. In a similar sense, Ti-rich (and hence Fe 3+ -poor) hemoilmenite lamellae are found to stabilize intergrown titanomagnetite [Garming et al, 2007].…”

Identification of magnetic Fe-Ti oxides in marine sediments by electron backscatter diffraction in scanning electron microscopy

“…The mineralogical identification of the chemically very similar Fe–Ti oxide phases TM and hemoilmenite using EBSD is important for the interpretation of ambiguous rock magnetic data, for example in identifying exsolution features or paragenesis of different mineral phases, such as in sample GeoB 6929‐6, 655. Low‐temperature rock magnetic analysis provided a strong indication of a Hilm mineral phase being present in the TM grains of this sample by showing a remarkable partial self‐reversal (Garming et al in rev.). Figure 5 shows the TM particles, which are derived from the Paraná volcanic province (Garming et al 2005).…”